Replace my furnace now or later? 80% vs 95%? HELP

Thanks for all of the replies - lots of good info! So it looks like I should for sure replace it - if the retrofit of the needed exhaust/supply isn't a major ordeal, I will plan to go with a 95% unit.

In regards to getting the correctly sized unit, I am wondering if you could consider the BTUs of the current (old) furnace and the way it acted in various temperature extremes, and then could gauge how much of a furnce I really need. I realize that doing a load calc get good info, but I am trying to see if more real-world / existing data would trump a hypothetical calculation.

The current furnace (1988) is 116k BTUs. I live in a 1600 ft2 tri-level house. The lower level is half finished, and half un-insulated crawl space. On the coldest winter days (-25 F), the furnace would run almost non-stop when the thermostat was set to 70.. On the other colder days, it would cycle on and off - the longest span of time that it would be off would be 15 minutes. Seemed like it would cycle on and off a lot.

Could I assume that the old furnace may have been 25% efficient? If so, could I assume that I was only getting/using 87k BTUs of the 116k BTUs, in which case, a 95k or 100k 95% furnace would be OK?

Another wrinkle in all this is that I just insulated the walls of my crawl, which I assume will lesson the need of the furnace.

In all, I am ok with getting a little bit more furnace than I need becuase we are contemplating a small addition (400 ft2) that will need to be heated.

Or, should I forget all this and do a proper load calc?

Lastly, are these install prices ($3600 for a 100k BTU 80%, $4400 for a 100k BTU 95%) out of line?

Replace my furnace now or later? 80% vs 95%? HELP

you would also have to have an accurate heat loss calc and know the comparable "degree days" to know the true seasonal efficiency over the heating season..ARE YOU KIDDING ME! gets a little complicated ay!

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Don't sweat it..you will definitely save .. BIG TIME with new Hi Eff Furnace. It will put a smile on your wallet.

Replace my furnace now or later? 80% vs 95%? HELP

To answer your other Q's a 100 K hi eff sound s good, esp considering add on..However ...and this depends a lot on your location but, IMO, I think those prices may be about 15 -20% higher than normal so, you may be able to easily get 10% off..Again there are many details we may not be aware of and those prices might be an absolute bargain. The most important key is to have a reputable installer!